Added Int32 decoding

[timotheeduran]

Swift documentation says

On 32-bit platforms, Int is the same size as Int32, and on 64-bit platforms, Int is the same size as Int64.

The actual code in the Int decode method is

switch a {
case let i as Int64:
    return Int(i)
case let i as Int:
    return i
default:
    throw MySQLCRUDError("Could not convert \(String(describing: a)) into an Int.")
}

When working on a 64-bit platform, the two first switch cases are equivalent and if a: Int32, an exception will be thrown.

The pull request adds a case for a: Int32 to ensure correct behavior when receiving a 32-bit Int on a 64-bit machine.